Class CheckpointManager

Vends hard-coded StoredBlocks for blocks throughout the chain. Checkpoints serve two purposes:

They act as a safety mechanism against huge re-orgs that could rewrite large chunks of history, thus
constraining the block chain to be a consensus mechanism only for recent parts of the timeline.

They allow synchronization to the head of the chain for new wallets/users much faster than syncing all
headers from the genesis block.

Checkpoints are used by the SPV BlockChain to initialize fresh
SPVBlockStores. They are not used by fully validating mode, which instead has a
different concept of checkpoints that are used to hard-code the validity of blocks that violate BIP30 (duplicate
coinbase transactions). Those "checkpoints" can be found in NetworkParameters.

The file format consists of the string "CHECKPOINTS 1", followed by a uint32 containing the number of signatures
to read. The value may not be larger than 256 (so it could have been a byte but isn't for historical reasons).
If the number of signatures is larger than zero, each 65 byte ECDSA secp256k1 signature then follows. The signatures
sign the hash of all bytes that follow the last signature.

After the signatures come an int32 containing the number of checkpoints in the file. Then each checkpoint follows
one after the other. A checkpoint is 12 bytes for the total work done field, 4 bytes for the height, 80 bytes
for the block header and then 1 zero byte at the end (i.e. number of transactions in the block: always zero).

checkpoint

Convenience method that creates a CheckpointManager, loads the given data, gets the checkpoint for the given
time, then inserts it into the store and sets that to be the chain head. Useful when you have just created
a new store from scratch and want to use configure it all in one go.

Note that time is adjusted backwards by a week to account for possible clock drift in the block headers.